Meningitis is an infection causing inflammation of the meninges, the protective membranes surrounding the brain and spinal cord. Because this inflammation occurs next to the central nervous system, meningitis is considered a medical emergency requiring rapid diagnosis and immediate care. The diagnostic process must quickly confirm inflammation, pinpoint the specific cause, and begin targeted treatment. A delay in diagnosis, particularly in bacterial cases, can lead to serious neurological damage or death. Diagnosis involves a careful physical exam, preliminary screenings, and a definitive laboratory test.
Initial Clinical Assessment and Triage
Diagnosis begins with a thorough assessment of the patient’s clinical history and symptoms. Medical professionals look for the classic triad of meningeal irritation: sudden high fever, severe headache, and neck stiffness (nuchal rigidity). This triad is not always present, especially in infants or older adults.
The physical examination includes specific maneuvers designed to elicit signs of meningeal irritation. Kernig’s sign is tested by attempting to straighten the patient’s leg after the hip and knee are flexed to 90 degrees. If this action causes pain or resistance in the back or neck, the sign is positive.
Brudzinski’s sign is checked by passively flexing the patient’s neck forward. A positive result occurs if the patient involuntarily flexes their hips and knees in response to the neck movement. While the presence of these signs strongly suggests meningitis, their absence does not reliably rule out the condition.
Preliminary Diagnostic Tools
Before the definitive diagnostic procedure, preliminary tests support the initial suspicion or rule out risks. Blood tests, including a complete blood count, check for an elevated white blood cell count, a general sign of infection. Blood cultures are also collected to detect bacteria growing in the bloodstream, which indicates systemic infection.
A blood glucose measurement establishes a baseline for later comparison with the cerebrospinal fluid (CSF) glucose level. Other markers like C-reactive protein (CRP) and procalcitonin (PCT) may be checked; high procalcitonin levels specifically suggest a bacterial cause. If bacterial meningitis is strongly suspected, broad-spectrum antibiotics are administered immediately after blood cultures are drawn, even before a definitive diagnosis is made.
Brain imaging, typically a Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) scan, is often necessary before the spinal tap. This is done especially if the patient shows signs of increased intracranial pressure, such as an altered mental state. The imaging checks for brain swelling, abscesses, or lesions that could make the lumbar puncture dangerous by risking brain herniation. If the scan is clear, the procedure can proceed safely.
The Role of the Lumbar Puncture
The lumbar puncture, or spinal tap, is the gold standard for confirming a meningitis diagnosis. This procedure obtains cerebrospinal fluid (CSF), the clear fluid that surrounds the brain and spinal cord. Analyzing the CSF provides direct evidence of infection or inflammation within the meninges.
The patient is positioned on their side with knees drawn up, or sitting and leaning forward, to maximize space between the vertebrae. After the lower back is sterilized and numbed, a thin needle is inserted into the subarachnoid space, usually between the L3 and L5 vertebrae. This insertion point is below the end of the spinal cord, minimizing nerve damage risk.
A small CSF sample is collected into sterile tubes for laboratory transport. As the needle is inserted, the opening pressure of the CSF is often measured. Elevated pressure is a significant finding, strongly suggesting bacterial or fungal meningitis. The procedure is prioritized unless there are contraindications, as CSF analysis provides the final confirmation of the disease.
Pinpointing the Type of Meningitis
The definitive classification of meningitis relies on laboratory analysis of the collected cerebrospinal fluid (CSF). This analysis focuses on three main components: cell count, protein levels, and glucose levels. The goal is to distinguish the cause of the infection—bacterial, viral, or fungal—which determines the required treatment.
A high white blood cell count (pleocytosis) indicates inflammation, and the cell type aids classification. Bacterial meningitis typically shows a very high number of white blood cells, predominantly neutrophils. Conversely, viral meningitis usually presents with a lower white blood cell count where lymphocytes are the predominant cell type.
The CSF glucose concentration is compared to the blood glucose level measured earlier. Since bacteria consume glucose, a low CSF glucose level (a CSF-to-blood glucose ratio less than 0.4) strongly suggests a bacterial or fungal infection. Viral meningitis usually results in a CSF glucose level that remains within the normal range.
Protein levels in the CSF increase during infection as the blood-brain barrier becomes more permeable. Protein levels are significantly elevated in bacterial meningitis, often above 1.0 g/L. Viral meningitis typically shows a moderate elevation. The CSF is also examined via a Gram stain and culture to identify the specific pathogen.